In modern motor vehicles, devices are being increasingly used which perform early detection of defects and malfunctions in various areas in the motor vehicle and signal defects and malfunctions to the driver. This includes, for example, the sensing of the tire air pressure in order to avoid defects or accidents which are due to an excessively low tire air pressure. In many of the systems which are already used for this purpose, in each case a tire module is arranged at each wheel, in particular in the interior of the tire. The tire module usually comprises at least one sensor for sensing the tire air pressure, if appropriate an assigned electronic evaluation system and a transmitting unit for transmitting the tire air pressure to a central processor unit on the vehicle.
Tire modules which are arranged in the interior of the tire in the region of the tread should be as lightweight as possible since they are otherwise destroyed by the forces occurring the tire or generate an imbalance of the tire. For this reason, as few elements as necessary are integrated into such tire modules. It is therefore known for a piezo-electric element to be arranged in a tire module which is arranged in the interior of the tire, which piezo-electric element is used both to detect the rotation of the tire and to supply power to the tire module.
For example, DE 44 02 136 A1, which is incorporated by reference, discloses a system for determining the operational parameters of vehicle tires, in which system a carrier unit, which is fitted with a sensor unit with at least one detector and one electronic evaluation system, is arranged integrated into the vehicle tire. A piezo-electric element, which serves to supply power to the system components and at the same time serves as a sensor for the rotation of the tire, is arranged on the carrier body. The system is integrated into the rubber of the tire, with the result that electrical power is acquired by virtue of the piezo-electric effect due to the hydrostatic pressure stresses which vary cyclically in the rubber of the tire.
DE 10 2004 031 810 A1, which is incorporated by reference, describes a sensor transponder having a piezo-electric element which is arranged in a vehicle pneumatic tire, wherein the piezo-electric element is arranged on the inside of the tire opposite the tread. The signal of the piezo-electric element is used here to determine the wheel speed, and the piezo-electric element provides all the power for supplying the electronic components of the sensor transponder and for transmitting data.